[taye]

What if your site is a graphic editing web app using a <canvas> renderer? By convention users should be able to copy and paste graphical objects created within the app as well as paste text or images copied from elsewhere.

If you use keyboard short-cuts in your UI to copy into your app's memory but not the system clipboard then you have no way to know whether the data the user wants to paste was copied from your app or somewhere else. If you only use the system clipboard and inform the user that copy/pasting is done with right-click or key-combos and not the browser's "Edit" menu, you can handle copy-pasting predictably.

This was the most reliable method I could think of. Yes the browser's "Edit" menu is a problem and there's no way to get around that.